Thin-film solar cells based on an absorber layer of chalcogenide compounds (CIGS, CdTe) are today among the most promising photovoltaic technologies due to their long-term ability to gain a foothold in mass commercial production as an alternative to conventional Si solar cells. Despite this success, the physical origin of the defects present in the thin films are still insufficiently elucidated, especially in the compounds of the chalcopyrite family Cu(In_{1x},Ga_{x})(S_{y},Se_{1y})_{2}. The research focuses on the identification and analysis of microstructural defects responsible for the electrical instability of chalcopyrite-based thin-film solar cells with a typical heterostructure arrangement ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se_{2}/Mo. The non-uniform polycrystalline nature of semiconductor materials in this complex multilayer structure requires a comprehensive analysis of electro-optical, structural and compositional properties associated with the actual morphology at the macroscopic, microscopic or even nanoscopic level. The observed predominant ohmic or non-ohmic current conduction in the dark transport characteristics was also reflected in the slope deviations of the excessive noise fluctuations, which were in the spectral domain exclusively in the form of flicker noise with dependency S_{i} ~ f^{1}. Spatially resolved electroluminescence based on stimulated photon emission by charge carriers injecting into the depletion region, not only showed a significantly inhomogeneous distribution of intensity in planar heterojunction under forward bias, but also revealed light emitting local spots in reverse bias due to a trap-assisted radiative recombination through the high density of defect states. Microscopic examination of the defect-related light emitting spots revealed rather extensive defective complexes with many interruptions through the layers, especially at the heterojunction CdS/Cu(In,Ga)Se_{2} interface. Besides, the high leakage current via these defective complexes subsequently led to a considerable local overheating, which caused a clearly observable structural and morphological changes, such as deviations in absorber layer stoichiometry due to Cu–In–Ga–Se segregation, Cu-rich and Ga-rich grains formation with an occurrence of Se-poor or Cu_{x}Se_{y} secondary phases regions, material redeposition accompanied by evaporation of ZnO:Al/i-ZnO/CdS layers together with the formation of Se structures on the surface around the defects. Within the research, analytical modelling of transport characteristics was implemented with parameters extraction of individual transport mechanisms to understand the non-ohmic shunt behaviour due to leakage current. In addition to the proper current path along the main heterojunction, the proposed model contains parasitic current pathways as a consequence of recombination-dominated charge transport or current conduction facilitated by multi-step tunnelling via high density of mid-gap defect states in the depletion region, ohmic leakage current caused by pinholes or low-resistance paths along grain boundaries in Cu(In,Ga)Se_{2}, or space-charge limited current due to metals diffusion from the ZnO:Al layer and grid Ag contacts through disruptions in i-ZnO/CdS layers.

This thesis is focused on the area of analysis and diagnostic of monocrystalline solar cells, using local light radiation. Main goals of this thesis are focused on explanation of generation and behavior microplasma in solar cell, in dependence of temperature and reversed bias voltage. As a next focus of this thesis is strong regions of microplasma sources, analyzed by scanning electron microscope and a detailed analysis of edge and surface structure is made. The influence of the environment (air pressure, level of vacuum and nitrogen gas) on the microplasma generation and behavior is observed in the vacuum chamber of electron microscope. The results from microplasma method are correlated with noise diagnostic method and on the base of these results are set a new thesis and mathematical equations for the defects behavior in different conditions. The outcomes of the research are shared with the manufacturer od silicon solar cells, Solartec s.r.o. company. It is the advanced diagnostic methods that allow improving the quality of the production process, through early detection of individual groups of defects.

Studying the microstructural features, optical, and electrical properties of the thin-filmperovskite solar cells (PSC) is the main objective of this thesis work. All the PSCs used in thisthesis work were prepared by spin coating assisted with gas quenching process and the samplesreceived from Interuniversity Microelectronics Centre (IMEC), Belgium.Microstructural and architectural details of the stagewise prepared PSCs were investigatedusing a Scanning Electron Microscope (SEM) - Focused Ion Beam (FIB) technique. With thereference to the given specification from IMEC, the SEM-FIB examinations of the as-preparedPSCs confirmed the presence of different layers such as hole transport layer (HTL), perovskitelayer, and electron transport layer (ETL). Further, the thickness of the perovskite layers wasmeasured and found to be 400 and 500 nm which validates the specification of the as-preparedsamples 1 and 2, respectively. The observed average grain size of the perovskite of the asprepared samples 1 and 2 are significantly different and the values are approximately 83 and169 nm, respectively. The average surface roughness values of perovskite layers (as-preparedsamples 1 and 2) and electron transport layer (as-prepared samples 3) were evaluated by atomicforce microscopy (AFM) and the values are 10, 19, and 12 nm, respectively. Furthermore, theconductive-AFM was performed to evaluate the electrical properties of the perovskite layers,and the results confirmed that the as-prepared sample 2 showed a higher mean current value of4.1 nA, than sample 1 resulted in 2.9 nA. The higher electrical performance of the as-preparedsample 2 could be correlated to the larger grain size, higher thickness, and higher surfaceroughness values of the perovskite layer.Moreover, the performance evaluation of a complete perovskite solar device with a similarconfiguration was evaluated between the as-prepared (newly fabricated) and the exposedsamples (tested under sunlight for ten weeks), and their behavior was studied. The optical andelectrical characteristics of the solar cell at the device level were examined with the help ofphotoluminescence (PL), electroluminescence (EL), and solar simulator techniques. The peakand fullwidth half maximum (FWHM) values of the PL emission spectra of the as-prepareddevice are in line with IMEC specification, whereas these values are slightly decreased for theexposed perovskite solar device. Also, during the EL examination, predominantly uniformluminescence was observed for the as-prepared device, whereas discontinuity in the emissionof electrons, and in some parts absence of luminescence-effect was observed for the exposedsolar cell. The current-voltage characteristics obtained from the solar simulator resultsconfirmed that the power conversion efficiency of the as-prepared device is at least 6 timeshigher than the exposed device. Based on the PL, EL, and PCE results it could be confirmedthat the perovskite solar cell exposed to sunlight for 10 weeks has started to degrade.

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京都大学 / 新制・課程博士 / 博士(理学) / 甲第24073号 / 理博第4840号 / 新制||理||1692(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 中家 剛, 教授 永江 知文, 准教授 WENDELL Roger / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Touch (or tactile) sensors are gaining renewed interest as the level of sophistication in the application of minimally invasive surgery and humanoid robots increases. The spatial resolution of current large-area tactile sensors (greater than 1 cm2) lag human fingers by over an order of magnitude. Using metal and semiconducting nanoparticles, a ~100 nm thick, large area thin-film device working on the principles of electron tunneling is self-assembled, such that the change in current density through the film and the electroluminescence light intensity are linearly proportional to the local stress. By pressing a United States 1 cent coin (and also a copper grid) on the device a well resolved stress image by focusing the electroluminescence light directly on CCD is obtained. Both the lateral and height resolution of texture are comparable to human finger at similar stress levels of ~10 KPa. The fabrication of the film is based on self-assembly of polyelectrolytes, and metal and semiconducting nanoparticles in a layered architecture. The polyelectrolyte layer functions as the dielectric tunneling barrier and the nanoparticles function as the base for tunneling electrons. The assembly of the device can be simplified by incorporating the functionality of the polyelectrolyte and the nanoparticles in a single composite medium. A non-micellar mineralization process for the synthesis of multifunctional nanocomposite materials is also reported as a possible building block for the assembly of tactile sensor. The non-micellar method results in the synthesis of monodisperse semi-conducting nanoparticles templated on polymer chains dissolved in solution at high yield. The monodispersity is achieved due to the beaded necklace morphology of the polyelectrolyte chains in solution where the beads are nanometer-scale nodules in the polymer chain and the nanoparticles are confined to the beads. The resultant structure is a nanoparticle studded necklace where the particles are imbedded in the beads. Multiple cycles of the synthesis on the polymer template yield nanoparticles of identical size, resulting in a nanocomposite with high particle fraction. The resultant nanocomposite has beaded-fibrilar morphology with imbedded nanoparticles, and can be solution cast to make electroluminescent thin film devices. The concept is further modified for synthesis of metal nanoparticles on polyelectrolyte templates with isolated beaded morphology. / Ph. D.

The term “electroluminescence” refers to light emission resulting from the application of an electrical bias. Electron tunneling across a biased, nanoscale junction can serve as the excitation source for photon emission. This effect is also mediated by the plasmonic environment of the junction, where a strong local field can enhance light emission by orders of magnitude. This dissertation presents measurements of electroluminescence from nanoscale gaps and single-molecule junctions. These measurements are made possible by a custom light emission detection system coupled to a scanning tunneling microscope break junction (STM-BJ) instrument. Conductance and light emission data are obtained simultaneously for thousands of junctions. Chapter 1 discusses molecular optoelectronics, a field at the intersection of plasmonic phenomena and molecular electronics, and introduces the STM-BJ technique for measuring molecular junctions. Chapter 2 describes the light emission detection setup that is operated in tandem with the STM-BJ instrument. Chapter 3 presents a study of Au tunnel junctions. This lays the groundwork for the plasmonics at play in these electroluminescent systems, detangling how gap size, electrical bias, and emission wavelength affect plasmonic enhancement. In Chapters 4 and 5, Au-molecule-Au junctions are investigated in some of the first experimental studies of single-molecule electroluminescence at ambient conditions. Chapter 4 uses light emission data from molecular junctions to estimate finite-frequency shot noise and uncover critical information about transmission characteristics. Chapter 5 presents one of the first examples of single-molecule strong light-matter coupling in an electroluminescent system, substantiated by spectroscopy data. This dissertation greatly expands on existing knowledge of plasmonic phenomena, particularly in relation to electroluminescent devices. Furthermore, it lays a strong foundation for single-molecule spectroscopy studies using the STM-BJ technique.